This invention relates to an improvement in a particular type of activated biomass process wherein the operating conditions of the system are controlled so as to maintain or to exceed a minimal phosphate removal rate, thus enhancing phosphorus removal from influent and simultaneously enhancing phosphorus uptake by biomass. The particular type of biomass process to which the present invention is applicable is one wherein a phosphate- and BOD-containing influent is mixed with recycled activated biomass in the initial contact step of the process, thereby producing a mixed liquor. This initial treating must be under anaerobic conditions, i.e., substantially free of oxygen and oxidizing agents and having a dissolved oxygen concentration of less than about 0.7 ppm. The biomass process further requires the subsequent treatment of the mixed liquor under oxic conditions thereby oxidizing BOD (Biological Oxygen Demand) in the mixed liquor and also causing storage of phosphates in the microorganisms. Subsequent to the oxic treatment, the mixed liquor is subjected to settling or separation whereby a purified supernatant liquid is separated from biomass, at least a portion of which settled biomass is returned to the initial contacting to provide the activated biomass admixed with the influent. The improvement of the present process is obtained by maintaining operating conditions in a biomass process of the type just described so as to enhance removal of phosphorus from the influent and incorporation thereof in the biomass in a high rate system.
Illustrative of the activated biomass process to which the present invention is applicable are those described by Marshall L. Spector in U.S. Pat. No. 4,056,465 entitled "Production of Nonbulking Activated Sludge", and in U.S. Pat. No. 4,162,153 entitled "High Nitrogen and Phosphorous Content Biomass Produced by Treatment of a BOD-Containing Material." In the first patent there is described a process wherein the mixed liquor is initially formed in an anaerobic zone, and such mixed liquor is passed to an oxic zone where it is subjected to oxidizing treatment, and the oxidized mixed liquor is subsequently passed to a settling zone from whence a portion of the activated biomass is recycled to the initial anaerobic zone. This patent also describes another variation of the basic type process wherein the mixed liquor is passed from the anaerobic zone to an anoxic zone (i.e. containing NO.sub.x.sub.-, but substantially no dissolved oxygen) positioned intermediate the anaerobic and oxic zones. In such variant there is also an internal recycle of mixed liquor from the oxic zone back to the anoxic zone, thus providing the NO.sub.x.sup.- in the anoxic zone. It is also disclosed in this patent that such process is effective to remove phosphorus from the influent and to cause the incorporation or storage of the phosphorus in the biomass.
This phenomenon wherein phosphorus is removed from influent and incorporated into biomass has been recognized by various people working in the area of wastewater treatment (see, for example, the references of record in the above mentioned patent).
The difficulties with which the art has been confronted in the past have included the phenomenon that the removal of phosphorus from the influent in the traditional oxygenating systems has not always been consistent, i.e. sometimes it worked, and sometimes it did not work. In other wastewater treating processes where the primary thrust has been the removal of nitrogen compounds from the influent, again phosphorus removal was not consistent, and it was even believed that phosphorus removal was directly related to nitrogen removal. Additionally, in all of these prior art processes, the period of treating required has always been quite extensive ranging upwards of 15 to 20 hours or more. In the processes of the type to which the present invention relates, such as those described in the above-mentioned Spector patent, while phosphorus removal from the influent was always present, the extent or rate of phosphorus removal varied significantly, and there was no means provided for controlling such process to insure enhanced phosphorus removal, high rate processing and a basis for optimizing design of the system.